It is known in the prior art that differential or pseudo-differential circuits have nodes that float at DC and prevent proper operation unless a method is provided to bias these nodes to a desired common mode. Therefore, it is known in the art to supply a DC bias voltage to a differential signal processing circuit, such as the inputs of an AC-coupled differential operational amplifier for example.
For example, a differential amplifier 102 may be AC-coupled to the output of a transducer 101, as schematically illustrated in FIG. 1A. The transducer output may be a single-ended voltage signal, referenced to ground. To interface this signal to the differential amplifier 102, which may have an input common-mode different from the transducer output's common mode, the transducer output 101 is AC-coupled to the non-inverting input 102A of op-amp 102, while the inverting input 102B of the op-amp 102 is AC-coupled to ground, and both inputs are provided (not shown) with nominally identical DC bias voltages.
FIG. 1B schematically illustrates a differential amplifier circuit 112 similarly configured to accept the single-ended or pseudo-differential output of transducer 101, process it, and produce a differential output signal.
A number of biasing circuits and methods are known in the art. Two simple biasing circuits are schematically illustrated in FIG. 2A and FIG. 2B, respectively. In each case, an amplifier's inverting and non-inverting inputs are biased by coupling them to a voltage source through two biasing resistors. An alternate biasing circuit is schematically illustrated in FIG. 2C, in which an amplifier's inverting and non-inverting inputs are biased by two voltage divider circuits coupled to a voltage source. A small-signal representation 270 of such a biasing circuit is schematically illustrated in FIG. 2D.